The catabolism of selected groundwater pollutants by a combined culture of Mycobacterium vaccae and a Rhodococcus sp. (strain R-22) was investigated. The M. vaccae-R-22 combined culture was five times more effective in mineralizing benzene than either organism alone. Mycobacterium vaccae oxidized benzene to phenol, and R-22 catabolized the phenol to cellular components and CO2. Benzene did not support growth of M. vaccae, R-22, or the combined culture. Optimization of ratios of the two species indicated that the maximum mineralization of benzene occurred at an initial ratio of 75% M. vaccae to 25% R-22. Cell fractionation of the combined culture after mineralization of [U-14C]benzene indicated that 10% of the benzene carbon was incorporated into cell material, and of this 45% was present in protein and 20% in nucleic acids. This suggested that minimally one species could utilize the products of benzene as a nutrient source. The M. vaccae-R-22 combined culture catabolized ethylbenzene and chlorobenzene without the accumulation of phenolic intermediates, which are inhibitory to M. vaccae's ability to degrade the parent compounds. This study demonstrates that defined mixed cultures may be useful in studying the effects of environmental pollutant degradation on microbial ecosystems and mineralization of these pollutants by the ecosystem.